The invention relates to a falling film evaporator that features the following criteria:                An external outer shell.        A given number of tubes the internal side of which has a liquid film flowing downwards.        Upper end devices for distributing the liquid to the individual tubes.        Lower end devices for collecting the residual liquid and the vapour.        A vapour extracting device for withdrawing the vapour forming inside the tubes.        A device for withdrawing the non-evaporated portion of the liquid inside the tubes.        Partitions that separate the internal chamber of the tubes and the hydraulic devices linked to the said chamber, for distributing and collecting the fluids from the space outside the tubes.        A device for feeding the gas/vapour mixture to the chamber enclosing the external side of the tubes.        A device for withdrawing the condensed liquid from the chamber enclosing the external side of the tubes.        A device for discharging the condensed vapours and the condensable gases from the chamber enclosing the external side of the tubes.        
The falling fill evaporator, therefore, is equipped with a header divided into segments in such a manner that the evaporator is also suitable for part-load operation. The invention is also related to a process suited for the operation of the said falling film evaporator and for transferring the heat released by the gas/vapour mixture during condensation and to be at least partly transferred to the liquid a portion of which evaporates during this cycle. Such a type of falling film evaporator operated in the mode described above is suitable for systems for heat recovery that are used, for example, in systems for the production of 1,2 dichloroethane (hereinafter referred to as EDC).
It is common knowledge that falling film evaporators permit a smooth heating of sensitive liquids. This is due to the high heat transfer coefficients of the inner side of the tube walls so that merely a small temperature difference is required between the outer wall side which as a rule is heated by a heating fluid and that of the liquid which forms a thin film on the inner side of the tube wall and evaporates. This feature, hence, also permits the utilisation of heating sources of lower value and the temperature level of which must only slightly exceed that of the liquid to be evaporated on the inner side of the tube wall.
This particularly applies if a higher heat transfer coefficient acts on the outer side of the tube wall because the heating fluid consists of condensable vapour. In this case the temperature difference between the heating fluid and the liquid to be evaporated may be as low as 3 K. In this context it is irrelevant what type of vapour is involved. Apart from water vapour, it is also possible to make use of vapours obtained in the course of the process, for instance, vapours from the distillation or chemically produced products withdrawn in the vaporous phase.
This is why falling film evaporators are becoming more and more profitable for heat recovery tasks in commercial scale plants. They are particularly suited for heating of the bottom of rectifying columns, such as in the EDC production outlined in EP 1 228 022 B1 and DE 36 04 968 A1.
In practical terms, however, falling film evaporators that are heated with a partly condensable fluid of a gas/vapour mixture exhibit the disadvantage of being inadequately suited for part-load operation. Whenever used as boiler for the rectification column bottom, this also encompasses poor controllability of the device as any reduction of the heat supply will inevitably lead to a temporary period of part-load operation because of the control action required on the evaporator. Difficulties are also encountered during start-up and shutdown operations.
The unfavourable properties encountered during part-load operation are due to the condensation behaviour of the heating fluid. During full-load operation the gas/vapour mixture is not hindered when entering the space enclosing the tubes via the controlled feeding device (hereinafter referred to as control valve) as the valve is in the 100% open position. The control action required for part-load operation of the evaporator entails a partial closure of the control valve and causes a pressure drop in the stream passing through the valve. The pressure drop thus obtained first leads to a partial pressure reduction of the heating vapours in the space outside the tubes and, consequently, to a reduction of the temperature governing the condensation of the liquid on the outer side of the tubes. In view of the very low temperature difference between the internal side and the outer side of the tubes, the evaporation temperature also decreases on the internal side, a phenomenon normally undesired because it will change the temperature conditions in the rectification column.
The second disadvantage is that the gas emerging from the gas/vapour mixture can be removed from the falling film evaporator with great difficulties only, i.e. if there is sufficient residual pressure. Depending on the upstream pressure and the pressure loss caused by the control valve it is necessary to deploy a vacuum pump for removing the gas. If the complete vapour cannot condense the vacuum pump must also withdraw the vapour portions.
A further problem arises if the gas emerging from the gas/vapour mixture develops in an explosive range. This particularly applies in the case of total condensation of the vapour. For example, when using a gas/vapour mixture from ethylene, oxygen and EDC, the said EDC being the major portion of the condensable component and with oxygen and ethylene prone to form an explosive mixture, the above-mentioned problem will occur during the condensation process. In this case it is therefore required to prevent with absolute security a total condensation of the EDC vapours.
The said problems are not only of a static nature but they also depend on the velocity of the control action taken. As a rule, quick-acting control actions are considerably more difficult to be stabilised than slow-acting control operations. Falling film evaporators of the conventional type and operational mode are particularly sensitive in this respect.